1. Field of the Invention
The present invent ion relates to a semiconductor device, and in particular, to a constant-voltage generating circuit which outputs a constant voltage as a reference voltage.
2. Description of the Prior Art
Japanese Laid-Open Patent Application No. 8-30345 discloses such a type of reference-voltage generating circuit (see FIG. 1), for example.
In the reference-voltage generating circuit in the prior art, shown in FIG. 1, a first transistor which is a depletion-type MOS transistor, a second transistor which is a MOS transistor, the conductivity type of which is the same as that of the first transistor, a source-follower circuit, a first voltage-supply terminal, a second voltage-supply terminal and a voltage-supply terminal for the source-follower circuit are provided. The drain of the first transistor is connected to the first voltage-supply terminal. The gate and source of the first transistor are connected to the drain of the second transistor. The source of the second transistor is connected to the second voltage-supply terminal. The gate of the second transistor is connected to the output terminal of the source-follower circuit or the terminal at which the output voltage of the source-follower circuit is divided. The input terminal of the source-follower circuit is connected to the connection point between the first transistor and the second transistor. The reference voltage can be provided from the output terminal of the source-follower circuit.
Further, in the reference-voltage generating circuit in the prior art, the source-follower circuit comprises a third transistor which is a MOS transistor, the conductivity type of which is the same as that of the first transistor, and a load of the source-follower circuit. The drain of the third transistor is connected to the voltage-supply terminal at which the voltage is supplied to the source-follower circuit. The gate of the third transistor is used as the input terminal of the source-follower circuit. A first terminal of the load of the source-follower circuit is connected to the source of the third transistor. A second terminal of the load of the source-follower circuit is connected to the other voltage-supply terminal at which the other voltage is supplied to the source-follower circuit. The connection point between the third transistor and the load of the source-follower circuit is used as the output terminal of the source-follower circuit.
Thereby, it is possible to provide the reference-voltage generating circuit with less consumed power, an adjustable temperature coefficient of the output voltage, a small output impedance, an output which can be provided externally of a semiconductor integrated circuit, and an output current can be provided. Further, it is possible to adjust the output voltage of the reference-voltage generating circuit, although such adjustment is not possible in the further prior art. Further, by using a sixth transistor (which is turned on and turned off externally of the reference-voltage generating circuit) in the load of the source-follower circuit, it is possible to provide the reference-voltage generating circuit in which it is possible to switch the current consumption and the output impedance when the state is changed between an operation state and a stand-by state.
Furthermore, the reference-voltage generating circuit in the prior art may be modified as follows: A plurality of source-follower circuits are additionally provided, all of the inputs of the thus-added source-follower circuits are connected to the connection point between the first transistor and the second transistor, and the outputs of the thus-added source-follower circuits are used as the reference-voltage output terminals separately.
Thereby, it is possible to provide a plurality of reference-voltage output terminals having no mutual interference, easily, and, also, without increasing the consumed current nor increasing the chip area, in comparison to the further prior art.
Further, the reference-voltage generating circuit in the prior art may be modified as follows: The source-follower circuit comprises the third transistor which is the MOS transistor, the conductivity type of which is the same as that of the first transistor, a source resistor and the load of the source-follower circuit. The drain of the third transistor is connected to the voltage-supply terminal at which the voltage is supplied to the source-follower circuit. The gate of the third transistor is used as the input terminal of the source-follower circuit. A first terminal of the source resistor is connected to the source of the third transistor. A second terminal of the source resistor is connected to the first terminal of the load of the source-follower circuit. The second terminal of the load of the source-follower circuit is connected to the other voltage-supply terminal at which the other voltage is supplied to the source-follower circuit. The connection point between the source resistor and the load of the source-follower circuit is used as the output terminal of the source-follower circuit.
Thereby, it is possible to provide the reference-voltage generating circuit in which it is possible to perform a stable operation using an input voltage higher than that of the further prior art.
Further, the reference-voltage generating circuit in the related art may be modified as follows: A seventh transistor, which is a MOS transistor, the conductivity type of which is different from that of the first transistor, or an eighth transistor, which is a MOS transistor, the conductivity type of which is the same as that of the first transistor, or both the seventh transistor and the eighth transistor are added to the source-follower circuit which comprises the third transistor, which is a MOS transistor, the conductivity type of which is the same as that of the first transistor, and the load of the source-follower circuit. When the seventh transistor is added, the connection between the voltage-supply terminal at which the voltage is supplied to the source-follower circuit and the third transistor is separated, the source of the seventh transistor is connected to the voltage-supply terminal at which the voltage is supplied to the source-follower circuit, and the drain of the seventh transistor and the gate of the seventh transistor are connected to the drain of the third transistor. When the eighth transistor is added, the connection between the other voltage-supply terminal at which the other voltage is supplied to the source-follower circuit and the load of the source-follower circuit is separated, the source of the eighth transistor is connected to the other voltage-supply terminal at which the other voltage is supplied to the source-follower circuit, and the drain of the eighth transistor and the gate of the eighth transistor are connected to the second terminal of the load of the source-follower circuit. The connection point between the drain of the third transistor and the load of the source-follower circuit is used as the output terminal of the source-follower circuit. The gate of the seventh transistor is connected to the drain of the seventh transistor. The output voltage to a constant-current circuit can be provided from the connection point between the third transistor and the seventh transistor, and, also, the other output voltage to the constant-current circuit can be provided from the connection point between the load of the source-follower circuit and the eighth transistor. The output voltages to the constant-current circuit are supplied to the gates of MOS transistors included in the constant-current circuit.
Thereby, it is possible to provide the reference-voltage generating circuit in which it is possible to freely adjust the temperature coefficient, and, also, it is possible to freely adjust the output current of the constant-current circuit.
However, in these reference-voltage generating circuits in the prior art, the reference output voltage is generated using the sum of the threshold voltage Vth of the first transistor and the threshold voltage Vth of the second transistor. Therefore, it is difficult to generate a reference output voltage close to the power-source voltage.
For example, it is difficult to generate a reference output voltage which is slightly lower than the power-source voltage, such as: EQU power-source voltage-0.1 (V)
for example.